add support for protocol-relative URLs

I typically include scripts from the Google CDN in my projects, e.g. `<script src="//ajax.googleapis.com/ajax/libs/angularjs/1.2.15/angular.min.js"></script>`, and their recommended practice is protocol-relative URLs, which help in situations where the client can choose to load resources over HTTP or HTTPS. I noticed grunt-appcache wasn't including the CDN files in the manifest, however.

This commit fixes a bug in `isAbsolutePath`, which returns false for an absolute URL like '//cdn.com/lib.js'. It just makes the http(s) optional when it tests the beginning of the path.